The development of an electroluminescent device using an organic material has been advanced with a view to putting the device into practical use as a high-performance flat panel having characteristics referred to as self light emission and high-speed response because a significant improvement in luminescent efficiency as compared to that of a conventional device using a single crystal of anthracene or the like has been achieved through the development of a device involving: optimizing kinds of electrodes with a view to improving the efficiency with which charge is injected from an electrode; and providing a hole-transporting layer composed of an aromatic diamine and a luminescent layer composed of an 8-hydroxyquinoline aluminum complex (hereinafter referred to as “Alq3”) as thin films for a gap between electrodes.
A constitution for additionally improving the efficiency of such organic EL device is obtained by appropriately providing the above basic anode/hole-transporting layer/luminescent layer/cathode constitution with a hole-injecting layer, an electron-injecting layer, or an electron-transporting layer. Known examples of such constitution include: an anode/hole-injecting layer/hole-transporting layer/luminescent layer/cathode constitution; an anode/hole-injecting layer/luminescent layer/electron-transporting layer/cathode constitution; an anode/hole-injecting layer/luminescent layer/electron-transporting layer/electron-injecting layer/cathode constitution; and an anode/hole-injecting layer/hole-transporting layer/luminescent layer/hole-blocking layer/electron-transporting layer/cathode constitution. The hole-transporting layer has a function of transmitting a hole injected from the hole-injecting layer to the luminescent layer, and the electron-transporting layer has a function of transmitting an electron injected from the cathode to the luminescent layer. The hole-injecting layer may be referred to as an anode buffer layer as well.
In addition, the following has been known. The hole-transporting layer is interposed between the luminescent layer and the hole-injecting layer, whereby a large number of holes are injected into the luminescent layer in a reduced electric field. Furthermore, an electron injected into the luminescent layer from the cathode or from the electron-transporting layer is accumulated on an interface between the hole-transporting layer and the luminescent layer because the hole-transporting layer extremely hardly permits an electron to flow in the layer, whereby luminescent efficiency increases.
The following has also been known. The electron-transporting layer is interposed between the luminescent layer and the electron-injecting layer, whereby a large number of electrons are injected into the luminescent layer in a reduced electric field. Furthermore, a hole injected into the luminescent layer from the anode or from the hole-transporting layer is accumulated on an interface between the electron-transporting layer and the luminescent layer because the electron-transporting layer hardly permits a hole to flow in the layer, whereby luminescent efficiency increases. The development of a large number of organic materials has been heretofore advanced in association with the function of such constituent layer.
Meanwhile, each of a large number of devices including a device provided with the hole-transporting layer composed of an aromatic diamine and the luminescent layer composed of Alq3 described above has utilized fluorescent emission. It should be noted that the utilization of phosphorescent emission, that is, light emission from a triplet excited state is expected to improve efficiency by a factor of about three as compared to that of a conventional device utilizing fluorescence (singlet). Investigation has been conducted into the use of a coumarin derivative or a benzophenone derivative in a luminescent layer for this purpose, but only extremely low luminance has been obtained. After that, investigation has been conducted into the use of a europium complex as an attempt to utilize a triplet state, but the investigation has not reached light emission with high efficiency.
It has been recently reported that the use of a platinum complex (T-1, PtOEP) enables the emission of red light with high efficiency (Nature, vol. 395, p. 151, 1998). After that, doping a luminescent layer with any one of iridium complexes (T-2, Ir(ppy)3) has significantly improved the efficiency with which green light is emitted (Appl. Phys. Lett., vol. 75, p. 4, 1999). Furthermore, it has been reported that each of those iridium complexes shows extremely high luminescent efficiency even when a device structure is additionally simplified by optimizing a luminescent layer (Appl. Phys. Lett., vol. 77, p. 904, 2000).
It should be noted that chemical formulae for T-1, T-2, and the like described above have been described in the following documents, so reference should be made to the documents. In addition, structural formulae and abbreviations for a host material, a guest material, and a compound generally used in an organic layer such as a hole-injecting layer or an electron-transporting layer have also been described in the following patent documents, so reference should be made to the documents. The abbreviations used in the following description without any notation are abbreviations generally used in this technical field, and are understood as meaning the abbreviations described in the following documents and the like.    Patent Document 1: JP-A-2002-305083    Patent Document 2: JP-A-2001-313178    Patent Document 3: JP-A-2002-352957    Patent Document 4: JP-A-2000-200684    Patent Document 5: JP-A-2003-515897    Patent Document 6: JP-A-10-25472    Non Patent Document 1: Appl. Phys. Lett., 77, 904, 2000
It is a carbazole compound CBP presented in JP-A-2001-313178 that is proposed as a host material in the development of a phosphorescent organic electroluminescent device. When CBP is used as a host material for a tris(2-phenylpyridine)iridium complex (hereinafter referred to as “Ir(ppy)3”) serving as a phosphorescent material capable of emitting green light, a charge injection balance is lost owing to the property of CBP with which a hole is easily allowed to flow and an electron is hardly allowed to flow. Then, an excessive amount of holes flow to an electron transportation side, with the result that the efficiency with which light is emitted from Ir(ppy)3 reduces.
A solution to the above problem is the provision of a hole-blocking layer for a gap between a luminescent layer and an electron-transporting layer. The hole-blocking layer efficiently accumulates a hole in the luminescent layer, whereby the probability of the recombination of the hole with an electron in the luminescent layer is increased, and an improvement in luminescent efficiency can be achieved. Examples of a hole-blocking material that has been generally used at present include 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as “BCP”) and p-phenylphenolate-bis(2-methyl-8-quinolinolato-N1,O8)aluminum (hereinafter referred to as “BAlq”). Each of the materials can prevent the occurrence of the recombination of an electron and a hole in the electron-transporting layer. However, a device lifetime is extremely short because BCP is apt to crystallize even at room temperature and its reliability as a material is poor. In addition, BAlq has a Tg of about 100° C., and hence a relatively good result of a device lifetime has been reported. However, BAlq does not have any sufficient hole-blocking ability, so the efficiency with which light is emitted from Ir(ppy)3 reduces. In addition, the number of layers in a layer constitution increases by one. The increase leads to a problem in that a device structure becomes complicated and a cost increases.
In addition, JP-A-2002-305083 discloses an organic EL device using a complex (—Ar1—Ar2—O—)nM, which is composed of a group having a nitrogen-containing heterocyclic ring Ar1 and an aromatic ring Ar2 and a metal M, as a host material that can be used instead of CBP, and a noble metal-based metal complex as a guest material in a luminescent layer. An enormous number of host materials are exemplified in the document; provided that a compound in which Ar1 represents a benzoxazole ring and Ar2 represents a benzene ring is exemplified as one of a large number of compounds. A compound in which M represents Zn and n represents 2 is also exemplified in the document, but the exemplification is limited to this compound. In addition, a large number of noble metal-based metal complexes each serving as a guest material are also exemplified. However, the exemplification is limited to the case where each of the metal complexes is used only in a device with a four-layer structure having a hole-blocking layer, so the document has not solved the above problems yet.
Meanwhile, 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (hereinafter referred to as “TAZ”) presented in JP-A-2002-352957 is also proposed as a host material for a phosphorescent organic electroluminescent device. However, a luminescent region is on the side of a hole-transporting layer owing to the property of TAZ with which an electron is easily allowed to flow and a hole is hardly allowed to flow. Therefore, depending on a material for the hole-transporting layer, the efficiency with which light is emitted from Ir(ppy)3 may reduce owing to a problem in terms of compatibility with Ir(ppy)3. For example, 4,4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (hereinafter referred to as “NPB”) that has been most frequently used in a hole-transporting layer because of its high performance, high reliability, and long lifetime has poor compatibility with Ir(ppy)3, so the material has a problem in that energy transition from Ir(ppy)3 to NPB occurs to reduce luminescent efficiency.
A solution to the above problem is the use of a material such as 4,4′-bis(N,N′-(3-tolyl)amino)-3,3′-dimethylbiphenyl (hereinafter referred to as “HMTPD”) that does not cause any energy transition from Ir(ppy)3 in a hole-transporting layer.
Appl. Phys. Lett., vol. 77, p. 904, 2000 reports that light emission with high efficiency can be provided by a three-layer structure in a phosphorescent device by using: TAZ, 1,3-bis(N,N-t-butyl-phenyl)-1,3,4-oxazole (hereinafter referred to as “OXD7”), or BCP as a main material for a luminescent layer; Ir(ppy)3 as a doping material; Alq3 in an electron-transporting layer; and HMTPD in a hole-transporting layer, and that the efficiency is excellent particularly in a system using TAZ. However, HMTPD is apt to crystallize and its reliability as a material is poor because HMTPD has a Tg of about 50° C. Therefore, a device lifetime is extremely short, and the commercial application of the device is difficult. In addition, the device has a problem in that the driving voltage of the device is high.
In addition, JP-A-2000-200684 discloses an organic EL device of a luminescent material containing a complex in which two molecules of 2-(2-hydroxyphenyl)benzoxazoles and one molecule of phenols bond to one molecule of Al. An enormous number of compounds are exemplified in the document, and a combination with a dope material is also exemplified as a host material. However, the exemplification is limited to the case where fluorescent emission is utilized.
In addition, JP-A-10-25472 discloses an organic EL device of a luminescent material composed of a complex in which four molecules of oxyphenyl-benzoxazole and Al—O—Al bond to each other. An enormous number of luminescent materials are exemplified in the document, and an example concerning doping is present in the document. However, the exemplification is limited to the doping.